Skill suggestion:Walk of the Shadowman

A skill similar to blaze from diablo 2 or spirit walk with severance in Diablo 3, something able to convert movement speed into an offensive property.
Maybe creating a shadow that charges towards the enemy,which acceleration is based on ones movement speed and which only deals full damage at max speed?

i think there was a skill or flask like that, but it's not able anymore, i don't know why